Ink jet printing is a relatively recent development. While a number of methods for utilizing this technique have been suggested, the most common techniques involve either subjecting a continuous stream of ink droplets to a static electric field (see U.S. Pat. No. 3,596,275), or periodically generating a series of droplets in response to electrical pulses to form a discontinuous stream with sufficient energy to traverse a substantially straight trajectory to a recording medium. (See U.S. Pat. No. 3,946,398).
Various types of ink formulations have been tried in an effort to optimize the jetability of an ink as well as to ensure satisfactory adherence and resolution on the substrate being used. For example, a typical ink formulation or printing medium for use on a printable substrate comprises base liquids, which can be aqueous or non-aqueous, a coloring agent such as a dye and/or pigment material, a bacteriocide (to inhibit biological growth), and, if needed, an additive to render the ink fluid susceptible to an electric or a magnetic field.
Many inks include the addition of water to the base liquids (see e.g., U.S. Pat. No. 4,021,252), but the addition of water may increase such problems as biological growth and corrosive properties. (See Ashley et al, "Development and Characterization of Ink for an Electrostatic Ink Jet Printer", IBM J. Res, Dev., (January 1977). If desired, salts such as sodium nitrite or lithium chloride may be used to increase electrical conductivity, or magnetizable particles may be added to increase magnetic susceptibility.
Properties such as viscosity, surface tension, conductivity, resistivity, specific gravity, and pH have been used to describe ink formulations and evaluate their suitability for use in the ink jet systems.
A number of problems have been noted in trying to formulate ink compositions. These problems include nozzle clogging, nozzle crusting, alterations in surface tension due to repeated exposure to the atmosphere during the jetting and recirculation of the ink fluid, print quality (including optical density and dot characteristics), and lack of adhesion of ink to the substrate. Many of these problems may be inter-related, e.g., alterations in surface properties of an ink caused by repeated exposure to the atmosphere during jetting and recirculation may cause nozzle clogging. Evaporation of a portion of the base liquid or absorbance of atmospheric moisture by the ink fluid may, for example, change the solubility of a particular pigment or dye in the ink fluid.
U.S. Pat. No. 3,994,736, discloses the use of polyhydric alcohols such as propylene glycol and ethylene glycol in the base liquid because of their ability to prevent the drying of the ink in the nozzle with subsequent clogging of the nozzle orifice. The polyhydric alcohols may be mixed with a lower monohydric alcohol to ensure rapid drying of the ink after deposition on the substrate to be printed.
It has been observed that surface tension parameters are important indications of the jetability of ink fluids. U.S. Pat. No. s4,021,252, for example, discloses the use of a primary solvent comprising a mixture of one or more aliphatic alcohols and water. The surface tension is described as "less than 40 dyne/cm.", and "preferably 30-35 dyne/cm." It will be appreciated by those skilled in the art that the addition of water, which has a surface tension of 73 dyne/cm., to ink fluids will alter the surface tension of such fluids.
In the context of non-aqueous inks having an affinity for water, there are especially troublesome problems with absorbance of atmospheric moisture during exposure of ink droplets to air during the jetting process. Since approximately 98% of the ink droplets may be recirculated, the cumulative effects of this addition of water on surface tension and coloring agent solubility can significantly alter the performance of such inks. More particularly, a change in coloring agent solubility for an ink jet fluid can lead to the precipitation of a material such as a dye or pigment out of solution, thereby resulting in clogging and crusting of the nozzle orifice. It would be desirable, therefore, to have a non-aqueous ink jet ink that is not affected by moisture and that does not experience a significant change in surface tension or undergo precipitation when exposed to moisture.
Short chain and very reactive borate esters have been used for their desiccant abilities as deicing additives in kerosene based jet fuel. (See U.S. Pat. No. 2,960,819). Certain types of borate esters have previously been used in some hydraulic fluids for their corrosion inhibiting properties. (See U.S. Pat. No. 3,738,941). Glycols and polyhydric alcohols are sometimes used in these formulations.
Borate esters, however, have never been used to formulate inks for use in ink jet systems as far as is known.